The major goal of this proposal is to elucidate the molecular mechanisms by which the polymerase and 3' to 5' exonuclease activities of E. coli DNA polymerase I, the prototype DNA polymerase, discriminate against errors during DNA synthesis. We will approach this goal by investigating the effects of some physiologically important parameters on the fidelity of DNA synthesis by: 1) investigating the role of base pair stability in error discrimination as well as the effects of neighboring base sequence on error frequency; 2) investigating the mechanism of base selection by the polymerase activity; 3) investigating the molecular basis of manganese mutagenesis by examining the effects of Mn++ on the base selection and editing capacities of the enzyme; 4) investigating the molecular basis of transition and transversion mutations and the factors which influence their relative frequencies; 5) probing the structures of the catalytic sites of the enzyme by the use of monoclonal antibodies. As a result of these studies we hope to better understand the molecular basis of mutagenesis, and thereby to gain insight into the mechanisms of somatic mutation and its role in malignancy as well as in the process of aging.